The present invention relates to an extrusion press head for the production of tubular strands of thermoplastic material which includes an annular space for the material to be extruded which is delimited at the outer circumference thereof by a housing and at the inner circumference thereof by a mandrel concentric with the housing. The extrusion press head further includes an annular piston axially movable in the annular space and at least one feed channel for feeding the material to be extruded into the annular space below the piston, which feed channel communicates with the annular space through an annular outlet end.
In a known extrusion press head of the above-mentioned kind there are provided two hollow cylindrical outlet ends for the feed channels which are separated only by thin bushings from each other and which terminate at the same side of the annular piston into a channel (DT-OS No. 2,100,192).
An extrusion press head is further known, with a single hollow cylindrical outlet end of the feed channel communicating with the aforementioned annular space. In this construction there are provided several webs between the mandrel and the housing for securing the mandrel to the housing, in which the webs are connected by an annular member with each other, and arranged circumferentially displaced from each other at two different diameters (German magazine "Kunstoff 66", year 1976, pages 15-17).
In these two known extrusion press heads there occurs often a disadvantageous relative movement between the streams of plastic material emanating from the two outlet ends. In order to avoid imperfections in the product to be formed, equal speeds of the two individual streams of material, formed in the feed channels, are necessary. The higher the speed of the material in the channels will be, the more difficult it will be to synchronize the flow resistance in the channels correspondingly.
In a further extrusion press head as disclosed in DT-Gbm No. 71 46 597, the material plasticized by an extruder is fed laterally toward the annular piston, whereby the latter, in the region where the outlet end of the extruder extends through the aforementioned housing, has a cutout in form of a pocket which in any position of the annular piston has to communicate with the outlet end of the extruder. The plastic material passing through the pocket will remain in part longer in the annular piston than the other plastic material fed from the extruder. This will result that the period of dwell of the material in the annular space and that of the material pushed out by the annular piston will be different and that the material which is fed first by the extruder into the extrusion press head will not leave the latter also ahead of the other material.